One known family of porous crystallline materials are the zeolitic materials, which are based on the 3-dimensional, four-connected framework structure defined by corner-sharing [TO4] tetrahedra, where T is any tetrahedrally coordinated cation. Among the known materials in this family are aluminosilicates that contain a three-dimensional microporous crystal framework structure of [SiO4] and [AlO4] corner sharing tetrahedral units, aluminophosphates that contain a three-dimensional microporous crystal framework structure of [AlO4] and [PO4] corner sharing tetrahedral units, and silicoaluminophosphates (SAPOs), in which the framework structure is composed of [SiO4], [AlO4] and [PO4] corner sharing tetrahedral units. Included in the zeolitic family of materials are over 180 different porous framework types, many of which have great commercial value as catalysts and adsorbents.
Recently a new family of porous materials has been synthesized that is based on [M(IM)4] tetrahedra in which IM is an imidazolate type linking moiety and M is a transition metal. These new materials are generally referred to as zeolitic imidazolate frameworks or ZIFs since the angle formed by imidazolates (IMs) when bridging transition metals is similar to the 145° angle of the Si—O—Si bond in zeolites. As a result, despite the infancy of this area of research, it has already been possible to synthesize ZIF counterparts of a large number of known zeolitic structures as well as produce porous framework types hitherto unknown to zeolites. Discussion of this research can be found in, for example, the following publications from Professor Yaghi and his co-workers: “Exceptional Chemical and Thermal Stability of Zeolitic Imadazolate Frameworks”, Proceedings of the National Academy of Sciences of U.S.A., Vol. 103, 2006, pages 10186-10191, “Zeolite A Imidazolate Frameworks”, Nature Materials, Vol. 6, 2007, pages 501 to 506, “High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture”, Science, Vol. 319, 2008, pages 939 to 943, “Colossal Cages in Zeolitic Imidazolate Frameworks as Selective Carbon Dioxide Reservoirs”, Nature, Vol. 453, 2008, pages 207 to 212, and “Crystals as Molecules: Postsynthesis Covalent Functionalization of Zeolitic Imidazolate Frameworks”, Journal of the American Chemical Society, Vol. 130, 2008, pages 12626 to 12627.
Much of this work on ZIF structures is summarized in U.S. Patent Application Publication No. 2007/0202038, the entire contents of which are incorporated herein by reference. In particular, the '038 application discloses a zeolitic framework, comprising the general structure: M-L-M, wherein M comprises a transition metal and L is a linking moiety comprising a structure selected from the group consisting of I, II, III, or any combination thereof:
wherein A1, A2, A3, A4, A5, A6, and A7 can be either C or N, wherein R5-R8 are present when A1 and A4 comprise C, wherein R1, R4 or R9 comprise a non-sterically hindering group that does not interfere with M, wherein R2, R3, R5, R6, R7, R8, R10, R11, and R12 are each individually an alkyl, halo-, cyano-, nitro-, wherein M1, M2, M3, M4, M5, and M6 each comprise a transition metal, wherein when the linking moiety comprises structure III, R10, R11, and R12 are each individually electron withdrawing groups. The '038 application also discloses that the zeolitic frameworks are useful as catalyst supports and as adsorbents for gases, particularly carbon dioxide.
In the zeolitic frameworks claimed in U.S. Patent Application Publication No. 2007/0202038, the metal species are all transition metals, typically tetrahedrally-coordinated divalent transition metals, especially Zn2+ and Co2+. The only example disclosed in U.S. Patent Application Publication No. 2007/0202038 that contains mixed-valence metals is ZIF-5, which has a non-porous framework comprising octahedrally-coordinated In3+ in addition to tetrahedrally-coordinated Zn2+.
In addition, a number of materials having framework structures comprising a metal cation and an anion which is either [B(IM)4]− or [Al(IM)4]− are known in the literature. However, most of these materials do not have a tetrahedral framework and/or appear to be non-porous structures.
For example, a cationic layered structure [PbB(IM)4](NO3) and its neutral iso-structure TIB(IM)4 were reported by Ziegler et al. in 2002 and 2004, respectively. See Ziegler et al. “Construction of a Functional Layered Solid Using Tetrakis(imidazolyl)borate Coordinating Anion”, Inorganic Chemistry, Vol. 41, 2002, pages 4984 to 4986 and Ziegler et al. “Lead and Thallium Tetrakis(imidazolyl)borates: Modifying Structure by Varying Metal and Anion”, Inorganic Chemistry, Vol. 43, 2004, pages 4272 to 4277. However, although Pb2+ and Tl+ in the aforementioned structures assume a coordination number of four, their coordination geometries severely deviate from tetrahedral. As a result the products reported by Ziegler et al. appear to be generally two-dimensional layered structures as distinct from zeolitic or zeolite-like open frameworks.
A compound formulated AgB(IM)4 was reported by Pettinari et al. in 2000 and a compound formulated CuB(IM)4 was reported by Pike et al. in 2005. See Pettinari et al. “Synthesis, Characterization and X-ray Structural Studies of Novel Dinuclear Silver(I) Complexes of Poly(azolyl)borate Ligands”, Inorganic Chimica Acta, Vol. 308, 2000, pages 65 to 72 and Pike et al. “Convenient Synthesis of Copper (I) Thiolates and Related Compounds”, Inorganic Chimica Acta, Vol. 358, 2005, pages 1331 to 1336. The silver and copper compounds were synthesized in methanol and water, respectively. Although the aforementioned formula are consistent with tetrahedral frameoworks, no information about the crystal structure or crystallinity of these two materials was reported. More importantly, the aforementioned formula were assigned to the as-synthesized materials based on elemental analyses. The fact that the two materials in their as-synthesized form do not contain guest species, e.g. solvent molecules, within the frameworks is a strong evidence of the materials being non-porous.
JP Patent Application Publication No. 2007087737 “Lithium Ion Conducting Material and Secondary Lithium Ion Battery Using It” discloses a series of lithium salts formulated Li+[M(Azo)4-n(Q)n]−, wherein M is either B or Al, wherein Azo is an azole residue or a substituted azole residue, wherein Q is a residue of a compound except for azoles, and wherein n is 0, 1, 2, 3. The preparative method for the lithium salts with n=0 is exemplified in the synthesis of LiAl(IM)4, wherein IM is imidazolate, and involves reacting LiAlH4 with imidazole. Elemental analysis of the as-synthesized material was reported to be consistent with the formula LiAl(IM)4. The fact that this material in the as-synthesized form does not contain guest species, e.g. solvent molecules, within the framework is a strong evidence of it being non-porous.
According to the present invention, a new series of porous crystalline materials has been synthesized which have a tetrahedral framework comprising a general structure, M1-IM-M2, wherein M1 comprises a metal having a first valency and particularly a monovalent metal selected from Li+, Cu+, and Ag+, wherein M2 comprises a metal having a second valency different from said first valency and particularly a trivalent element selected from B3+, Al3+, and Ga3+, and wherein IM is imidazolate or a substituted imidazolate linking moiety. Such materials offer new opportunities for catalytic applications since, for example, Cu+, Ag+, Al3+ and Ga3+ exhibit different chemical behaviour than the divalent transition metals typically used for ZIF synthesis. Moreover, by employing low atomic weight elements Li and B, rather than transition metals, it should be possible to produce adsorbents with improved gas uptake on a gravimetric basis.